Is Floette's "Flower Veil" Ability stackable, such that if you have more than one in play it adds +20 for each "Flower Veil"?
Yes, that is correct.
Source: Flashfire FAQ (2014-05-08), TPCi Rules Team (2014-05-08)

Relevant cards

Note: All applicable cards may not be listed below.
XY2_64